I propose to study the sequential neuronal maturation (cortical ontogenesis), the neuronal architecture and the fibrillar-neuronal organization of the cerebral cortex of the normal and abnormal fetuses, prematures, newborns, and young infants. To study the development and structural organization of the various layers which form the cerebral cortex, I have already started an in depth analysis of Layer I of the human cerebral cortex. These studies will be carried out primarily with the Rapid Golgi and in addition, with other routine neuropathological methods. Three main objectives are pursued in this investigation: 1) to establish control patterns and timetables of normal events in the sequential maturation and progressive structural organization of the human cerebral cortex in the course of prenatal and early postnatal development. Studies have been started on the development and structural organization of Layer I, 2) to utilize these observations as control data in the study of the abnormal structural organization of the cerebral cortex. 3) to study the cerebral cortex of a) infants with chromosomal abnormalities known to be associated with mental retardation b) of the low-birth weight and single live born premature, the causes of prematurity and its high mortality rate and the high-risk for mental retardation, and c) of infants with congenital malformations involving the cerebral cortex. In addition, I propose to study the development (ontogenesis and phylogenesis) and the structural organization of the mammalian cerebral cortex in experimental animals (the cat and the hamster) and in comparative studies with man.